A conventional antenna duplexer is disclosed in Japanese Patent Laid-Open Publication No. 2002-330057 includes surface acoustic wave filters different from each other. Each of the filters includes comb-shaped electrodes provided on a single piezoelectric substrate.
Such a conventional antenna duplexer requires isolation between the surface acoustic wave filters for allowing the surface acoustic wave filters to exert their characteristics effectively. To meet this requirement, the arrangement of these filters on the piezoelectric substrate is optimized, and a groove is provided in the substrate between a transmitting filter and a receiving filter so as to increase the isolation between the filters.
In the conventional antenna duplexer, the filters may be coupled with each other due to a surface acoustic wave coupling for coupling the filters by surface acoustic waves on the piezoelectric substrate and to a high-frequency coupling form coupling the filters by a transmitting signal and a received signal interfere with each other. The groove provided in the substrate cannot suppress the high-frequency coupling, hence not improving the isolation between the surface acoustic wave filters.